Frequency and timing synchronization and error correction in a satellite network

ABSTRACT

An outgoing transmission carrier frequency at, for example, a remote terminal, may be adjusted based on a feature rate of an incoming transmission. The adjustment may compensate for error in the outgoing transmission carrier frequency. The incoming transmission may be from a satellite or hub, for example. The outgoing transmission may be the signal that is transmitted by an antenna, or the outgoing transmission may be converted to be at another carrier frequency such as a higher carrier frequency for ultimate transmission by an antenna. Also, a remote terminal may be synchronized to a satellite network with or without knowledge of the remote terminals&#39; geographical location and with or without time stamps.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. ProvisionalPatent Application Ser. No. 60/472,727, entitled “Satellite Frequencyand Timing Synchronization and Correction,” filed May 23, 2003,incorporated by reference herein as to its entirety.

BACKGROUND OF THE INVENTION

[0002] The most common topology of a satellite network is the startopology, in which a central hub broadcasts toward a plurality of remoteterminals (such as very small aperture terminals, or VSATs) via aforward time-domain multiplexed (TDM) carrier, and the remote terminalscommunicate with the hub by return channel bursts. There are many commonmultiple access schemes, the choice of which involves a dilemma and awell-known trade-off. Contention-based random access (CRA) solutions,such as Slotted Aloha, tend to have short response-time per transaction.However, the throughput is limited theoretically to about 36%utilization of the available channel. Moreover, in order to achieveacceptable average delays (that are dominated by collisions and the needfor re-transmissions), the practical load should be about 30%. A morebandwidth efficient access scheme would provide a collision-freeapproach. A simple fixed time-domain multiple-access (TDMA) scheme,where slot allocations are pre-determined, is not efficient in thetraffic regime where the terminal return link needs are highly dynamicand variable.

[0003] A more popular approach is a “reservation-based” solution, inwhich the hub allocates time slots and frequencies to remote terminalsaccording to the momentary needs of the remote terminals. This solutionis also referred as a “reservation” system. In a reservation system, theremote terminals each transmit over a return control channel a “requestfor allocation” message to the hub whenever there is a need to transferdata to the hub. However, the repeated transmission of request forallocation messages imposes significant delay and added transmissionoverhead. A good design would try to keep the request for allocationmessages very short on a special short-slots structure.

[0004] Multiple-access VSAT networks often involve complicated time andfrequency synchronization aimed to align the burst transmissions intothe pre-designed frequency and time slot scheme. It is particularlycomplicated to align the first transmission of a remote terminal when,for example, the remote terminal has just been powered up.Traditionally, synchronization for both burst-slot timing and frametiming is accomplished via synchronization time-stamps that are sent viathe forward channel. This involves some over-head in the Forwardchannel. Moreover, it involves costly and complicated hardware in thecentral hub, for accurate insertions and stamping of these time stampswithin the forward channel.

[0005] Moreover, existing reservation networks cannot deal with initialfrequency errors that are more than a fraction of channel frequencyspacing, and cannot tolerate burst-timing errors that are larger thanslot time guard bands. To allow for proper timing, the remote terminalsare typically provided with information about their own geographiclocation and satellite location (to determine the distance between theterminal and the satellite), as well as time-stamps. However, thegeographic information requirement makes it difficult to provide formobile remote stations, and makes setup more difficult and timeconsuming.

[0006] In addition, because there are typically relatively fewsatellites and hubs in a satellite network as compared with remoteterminals, and because many remote terminals are intended to be sold toindividuals, remote terminals are often manufactured with less expensiveelectronics than in satellites and hubs. For example, each of thesatellites, hubs, and remote terminals has circuitry that generates itsoutgoing carrier frequency or frequencies. Although such circuitry maybe extremely accurate in the satellites and hubs, the less expensivecarrier frequency generation circuitry in the remote terminals may besubject to drift and/or other frequency error. Such errors can beproblematic in that the various carrier frequencies used in a satellitenetwork are often close together. Any substantial frequency error canresult in miscommunication.

SUMMARY OF THE INVENTION

[0007] There is a need for ensuring that transmission frequency andtiming are efficiently synchronized and managed among the varioussatellites, hubs, and remote terminals.

[0008] Therefore, aspects of the present invention allow for eliminatingthe periodic distribution over the forward channel of synchronizationdata (e.g., time stamps), for slot and/or frame synchronization of thereturn channel. Instead, slot and/or frame synchronization informationis re-constructed at the remote terminal from the re-constructedreception data clock, its derivatives—byte clock, the MPEG Sync pulse (apulse that occurs at the beginning of each 188-byte MPEG block), etc.,and/or the re-constructed symbol clock (derived from, e.g., the baudrate of the received data) and its derivatives, and/or by proper bursttiming error and burst frequency error feedback information from thehub. This is possible by using receivers configured to be able toreceive initial bursts that are not necessarily aligned with time-slots,and by using timing feedback correction messages sent to the remoteterminals from the hub.

[0009] According to further aspects of the present invention, efficientusage of satellite bandwidth resources may be maintained, while theincorporation of low-cost terminals with large initial frequency erroris supported. This is possible by using receivers configured to be ableto receive initial bursts that are not necessarily aligned with channelfrequencies, and by using frequency feedback correction messages sent tothe remote terminals from the hub. Furthermore, the system may supportunknown geographic location of remote terminals, thus simplifyinginstallations, and easing the support of transportable terminals.

[0010] Still further aspects of the present invention allow largeinitial burst-timing errors, and/or large initial burst frequencyerrors. Such initial frequency errors may, for instance, be limited onlyby the entire return-channel total bandwidth allocation. Thus, low costtransmitters, such as dielectric resonant oscillator (DRO)-basedtransmitters, with large initial frequency offsets, may now be morepractically utilized in remote terminals.

[0011] Still further aspects of the present invention allow remoteterminals to successfully operate even without prior knowledge of theremote terminal's geographical location or the delay of its transmissionto the satellite or to the hub. This is because initial burst-timingalignment is not required. This greatly simplifies installation ofremote terminals and increases their robustness. This feature ofindependence from geographical location may also be useful forincorporating mobile or transportable remote terminals into a satellitenetwork.

[0012] There is also a need to reduce the potential for frequency errorin outgoing transmissions from remote terminals. Although aspects of thepresent invention are discussed with regard to remote terminals, suchaspects may be used in any communications equipment such as insatellites or hubs.

[0013] Therefore, further aspects of the present invention are directedto adjusting an outgoing transmission carrier frequency at, for example,a remote terminal, based on a feature rate of an incoming transmission(such as the baud rate) that serves as a global clock reference for allthe remote terminals. The incoming transmission may be from a satelliteor hub, for example. The outgoing transmission may be the signal that istransmitted by an antenna, or the outgoing transmission may be convertedto be at another carrier frequency such as a higher carrier frequencyfor ultimate transmission by an antenna.

[0014] Further aspects of the present invention are directed to countinga predetermined number of pulses associated with the incomingtransmission, and counting how many pulses of a locally-generatedreference clock occur within the time period for counting the pulses ofthe incoming transmission. The pulses associated with the incomingtransmission may be from a clock reconstructed from the incomingtransmission. The reconstructed clock may have a pulse rate equal to, orotherwise related to, the feature rate of the incoming transmission. Bycomparing the actual number of reference clock pulses with an expectednumber of reference clock pulses, the error in the reference clock,compared to the reconstructed clock, may be determined and compensatedfor. The reference clock may then be used to generate the outgoingcarrier frequency.

[0015] Still further aspects of the present invention are directed tocounting a predetermined number of pulses of the reference clock, andcounting how many pulses associated with the incoming transmission occurwithin the time period for counting the pulses of the reference clock.Again, by comparing the actual number of incoming transmission-relatedpulses are counted with an expected number of pulses, the error in thereference clock, compared to the reconstructed clock, may be determinedand compensated for. The reference clock may then be used to generatethe outgoing carrier frequency.

[0016] These and other aspects of the invention will be apparent uponconsideration of the following detailed description of illustrativeembodiments. Although the invention has been defined using the appendedclaims, these claims are illustrative in that the invention is intendedto include the elements and steps described herein in any combination orsubcombination. Accordingly, there are any number of alternativecombinations for defining the invention, which incorporate one or moreelements from the specification, including the description, claims, anddrawings, in various combinations or subcombinations. It will beapparent to those skilled in the relevant technology, in light of thepresent specification, that alternate combinations of aspects of theinvention, either alone or in combination with one or more elements orsteps defined herein, may be utilized as modifications or alterations ofthe invention or as part of the invention. It is intended that thewritten description of the invention contained herein covers all suchmodifications and alterations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The foregoing summary of the invention, as well as the followingdetailed description of preferred embodiments, is better understood whenread in conjunction with the accompanying drawings, which are includedby way of example, and not by way of limitation with regard to theclaimed invention.

[0018]FIG. 1 shows an illustrative satellite communication system.

[0019]FIGS. 2, 3, and 6 are timing diagrams showing various frameportions.

[0020]FIG. 4 is an illustrative flowchart showing communication betweena remote terminal and a hub.

[0021]FIG. 5 is an illustrative functional block diagram of a remoteterminal.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0022]FIG. 1 shows an illustrative satellite network including asatellite 103, a hub 101, and a plurality of remote terminals 102 suchas, but not limited to, VSATs. The hub 101 and the remote terminals 102as shown embody a two-way data network in a star configuration. Messagesfrom the hub 101 to the remote terminals 102 may be sent via a TDMsignal on a forward or outbound channel that is broadcast to some or allof the remote terminals (“multicast” messages). Messages to specificones of the remote terminals 102 (“unicast” messages) may also bemultiplexed along with the “multicast” messages. The hub 101 maytransmit via a standard DVB-S carrier, and may include a local DVB-Sreceiver that receives its own outbound transmission. The output datafrom the DVB-S receiver may be a standard MPEG transport stream.

[0023] The data from the remote terminals 102 to the hub 101 may be sentas a series of burst transmissions. Multiple access to the sharedfrequency-time resources may be implemented by allocating certain timeslots and/or frequencies to the bursts, so that multiple access ofmessages from multiple remote terminals 102 will preferably not collideor otherwise interfere with one another. The bursts may be organizedinto a plurality of time slots, which may further be organized into aplurality of frames. The return link shared resources may be accessed ina TDMA or frequency and time division multiple access (FTDMA) manner. Inthe embodiment disclosed herein, the common outbound carrier is a DVB-Sstandard carrier (ETS 300 421), and the data is organized over MPEGframes containing 188 bytes. However, other data formats may be used.

[0024] As explained below, a remote terminal may re-construct slotand/or frame timing synchronization. There are two potential operatingmodes. In a first operational mode, time stamp messages may betransmitted from the hub to the remote terminals via the outboundcarrier. This has some benefits, as will be seen later. A tradeoff isthe overhead used by the time stamps. In a second operational mode, timestamps may be omitted altogether.

[0025] A remote terminal 102 such as a VSAT may transmit an initialburst to the hub 101. The burst may be an initial burst because theremote terminal has just been powered up, reset, or for other reasons.In any event, at this state the remote terminal is not necessarilysynchronized with the hub in time and frequency. Especially where theremote terminal is not programmed with its geographical location, thetiming may not be expected to be synchronized with the hub. Thus, thereneeds to be flexibility by the hub to receive the initial burst from theremote terminal regardless of the time and frequency of the initialburst. It can be expected, however, that the initial burst will fallsomewhere within a known frequency range. The hub may thus have one ormore receivers that are configured to be able to receive a data burst atany frequency within a known frequency range, at any start time. Thereceivers may be triggered to receive the burst data in response tosensing the start of transmission energy from the remote terminal.

[0026] We will now address techniques wherein the remote terminal 102transmits the initial burst prior to specific correction from the hub101. We will refer to this initial burst transmission as a link connecttransmission. Referring to FIG. 4, the remote terminal 102 may bepowered up or reset (step 401), and may transmit an initial link connectburst transmission (step 402) to the hub 101, requesting access tonetwork resources. The initial link connect burst transmission may be ofany arbitrary frequency and timing, within a known frequency range. Thisis because the remote terminal 102 may have significant frequency errorwhile transmitting, and the remote terminal may further not know thetransmission distance (and thus propagation time) between the remoteterminal 102 and the satellite 103 at the moment of transmission. Thelink connect burst transmission may be a random access transmission,operating in un-slotted Aloha mode, both in time and in frequency. Whenthere are no time stamps, the local slot and frame synchronizations arearbitrary, and the remote terminal 102 cannot be assured that it willalign its transmission with a slot, much less a particular pre-assignedslot. The first link connect transmission frequency may have a largefrequency uncertainty of +/−F.

[0027] The hub 101 may receive the link connect burst transmission (step403) regardless of the frequency and timing. To receive the link connectburst, a single receiver or a bank of multiple receivers may be employedin the hub 101 that can receive all burst transmissions within thefrequency bandwidth of +/−F. To accomplish this, each receiver in thereceiver bank may be configured to detect any burst transmission with afrequency error of at least up to half of the frequency channel spacing.Each receiver in the hub 101 may further be configured to successfullyreceive link connect bursts at any arbitrary burst-start timing within aslot. If the first link connect transmission is not to interfere withnormal inbound data traffic, then at least 2*F of frequency bandwidthshould be allocated for the first link connect transmissions only, andnot be used for normal data traffic.

[0028] For instance, referring to FIG. 2, a plurality of frames areshown in which one or more portions of a frame may be allocated ascontrol channels for link connect bursts. Each frame includes aplurality of slots. Each frame/slot is also divided across a frequencyrange into a plurality of frequency channels. The hub 101 and the remoteterminals 102 communicate within the time/frequency domain defined bythe slot, frames, and frequency channels. Assume, for example, that theinbound time slot duration at the hub 101 is designed to be the durationtime of transmitting some integer X MPEG-frames by the outbound carrier.The frame may include some integer Y of time slots. A counter at the hubmay count X MPEG transmissions outbound, for extracting hub slot timing,and may count Y slots for hub frame timing. These timing pulses may bedistributed to all hub receivers.

[0029] Optionally, time stamps may be transmitted by the hub 101 via theoutbound carrier. According to U.S. Pat. No. 6,091,742 to Bassat et al.,issued Jul. 18, 2000, and incorporated by reference herein as to itsentirety, time stamps may be transmitted in an un-known multiplexingdelay environment, such as IP encapsulation, via a DVB-S TDM carrier,and random delay may be corrected in accordance with informationregarding a hub counter state when the previous time-stamp message wasreceived at the Hub (for instance, by a local DVB-S receiver).

[0030] At the remote terminal 102, the DVB-S outbound is received, andlocal counters may count the received X MPEG messages to obtain firstlocal slot timing and Y slots to obtain local frame timing. If timestamps are not transmitted outbound from by hub 101, these countersserve as the initial slot and frame timing, although the initial slotand frame timing is arbitrary.

[0031] However, if time stamps are transmitted by the hub 101, thereception of the time stamps and the added information of the time ofreception (X and Y values) of the previous time stamp may allow theremote terminal to re-phase the X and Y local counters to synchronizewith the reception of the time-stamps. This operation is possiblebecause the remote terminal 102 can measure its own X and Y timing ofreception of the previous time stamp and can compare it to theinformation that is received from the outbound about the X and Y timingof reception of the previous time stamp at the hub. Then, phasecorrection of the local X and Y counters may be performed by adding tothe local counters the modulus of the difference in the counters.

[0032] When time stamps are transmitted within the outbound carrier, theX and Y counters at the remote terminal may track the hub slot and framecounters. However, the remote terminal time reference counters will bedelayed by D′, relative to hub timing, due to the delay of the outboundpass (as the transmission goes from the hub 101 toward the satellite103, and from the satellite to the remote terminals 102, at about lightspeed). In a system where time stamps are not present, still the localslot and frame timings are at the correct speed, and only the time delaybetween the remote terminal timing and the hub timing is initiallyrandom and un-known to the remote terminals. Of course, when the hub 101sends acknowledge reports to the remote terminals 102 on their timingerrors (as discussed below), their timing may be corrected, regardlessof whether time stamps are used.

[0033] Assume, for example, that the time-frequency capacity availablefor a satellite network is a two-dimensional rectangle with time as onedimension and frequency as the other. Then, referring to FIG. 3, portion201 in frame X (and corresponding portion 202 in frame X+1, and so on)may be allocated as one or more link connect zones over which the remoteterminals 102 transmits their link connect burst transmissions. Theallocated control channels may straddle frame borders, such as doportions 301, 302, and 303. In a system that does not incorporate timestamps in the outbound, so that frame timing is completely unknown—thistwo-dimensional space for a link-connect should preferably include afrequency range of at least 2*F along the frequency axis, and preferablythe whole frame along the time axis (for example, the allocatedfrequency band 601 in FIG. 6). However, when time stamps are used, theminimal required time window W for the initial link-connect may bereduced, e.g., to about 40 milliseconds within a frame even withoutspecifying a geographical location of the remote terminal, and to about,e.g., 2 milliseconds when geographic location information is provided.

[0034] In addition to the link connect allocated zone and the normaldata traffic zone, there may further be a zone for control. This controlzone may be used for, e.g., allocation requests, health check, and/orgeneral control. The control zone may be multiple-accessed, such as byFTDM or using a contention-based approach (i.e. Slotted Aloha).

[0035] Continuing with the description in connection with FIG. 4, thehub 101 may analyze the link connect burst transmission (step 404) todetermine what the frequency and timing of the link connect bursttransmission are. Next, the hub 101 may send correction data to theremote terminal 102 indicating how much the remote terminal 102 needs tocorrect its timing and/or frequency (step 405). Thus, acknowledgemessages from the hub 101 towards the remote terminal 102 may providethe remote terminal 102 data with content that is associated with, orotherwise specifies, the burst timing correction and/or the frequencycorrection that is needed. For example, the correction data may indicatea particular amount of frequency and/or a particular amount of time thatthe remote terminal 102 should adjust future transmissions by. By usingthe correction data, the remote terminal 102 may ensure that furtherinbound transmissions are be aligned with the slot and frame timing aswell as the frequency grid.

[0036] In response to receiving the correction data (step 406), theremote terminal 102 may correct its timing and/or frequency (step 407).The remote terminal 102 may further send allocation requests and/orhealth-check indications from time to time using the control zone (step408). Access to the control zone may be, e.g., contention-based and/or aTDM structure. The control bursts may be acknowledged with timing and/orfrequency correction data, allowing the remote terminal 102 to track thecorrect timing and/or frequency grid. Thus, for example, if the remoteterminal 102 needs to transmit new data, it may send an allocationrequest in the control zone (step 408). The hub 101 may receive theallocation request (step 409) and allocate some capacity of slots andfrequencies for the transfer of that data in reservation mode. The hub101 may further analyze the timing and/or frequency of the allocationrequest (or health check) and generate correction data (step 410). Thehub 101 may then transmit an indication of the allocated slots andfrequencies, along with the correction data, to the remote terminal(step 411). Then, the remote terminal 102 may receive the correctiondata and use it to correct, as needed, the timing and/or frequency ofits subsequent transmissions (steps 412 and 413). In this way, thetiming and/or frequency synchronization of the remote terminal 102 maybe maintained over time. One channels are allocated to the remoteterminal 102, the remote terminal 102 may use the allocated capacity totransfer data bursts (step 414). The remote terminal 102 may furtherpiggy-back additional allocation requests on those data bursts.

[0037] This process of sending correction data may continue with eachsubsequent data burst. Alternatively, the correction data may be sentperiodically or only in connection with the initial data burst. Insummary, once the link connect burst from the remote terminal 102 isreceived by the hub 101, the hub 101 may acknowledge with frequencyand/or burst timing correction data. This allows the next remoteterminal inbound transmission to align with the frequency grid and withthe slot and frame timing. Also, because of frequency and/or time drifts(e.g., from Doppler effects and path distance variations due to smallperturbations in satellite locations), the remote terminal 102 mayinitiate an inbound transmission once in a while even if it does nothave a need to transfer data to the hub 101, just for the sake ofreceiving updated frequency and timing corrections. We refer thistraffic as “health check” bursts.

[0038] When the remote terminal 102 needs to transfer data, the remoteterminal 102 may request capacity allocation from the hub 101. Theremote terminal 102 may use a control channel for such a capacityrequest, unless it has already an allocation. The remote terminal 102may piggy-back the allocation request onto a data transfer. The controlchannel may be used not only for allocation request but also for healthcheck traffic and/or for general monitoring and control (M&C).

[0039] When time stamps are being sent and used, more useful informationabout the slot and frame timing is available to the remote terminals102. However, the link connect burst may still reach the hub 101 delayedby unknown delay, since the distance of the satellite transmission hop(i.e., the distance between the remote terminal 102 and the satellite103, and/or the distance between the satellite 102 and the hub 101) maybe unknown due to the remote terminal 102, as the terminal may not knowits specific geographical location.

[0040] Traditionally, in VSAT networks, the geographical location ofeach remote terminal 102 would be provided to the remote terminal, andthe remote terminal 102 would transmit at a slot timing that would alignthe burst reception at the hub 101 with the slot and frame timing bytaking into account the at least partially known satellite transmissionhop distance. However, this operation can only partially succeed, sinceas the exact calculations of transmit burst timing will have to takeinto account not only the location of the remote terminal and thegeneral location of the satellite, but also the precise momentarylocation of the satellite at a given point in time. The satellite 103typically has a perturbation of location that cannot be ignored (e.g.,on the order of about +/−1 millisecond when translated to delayperturbation). Moreover, the need to specify the geographical locationto the remote terminal 102 adds to installation complexity, and isspecially complicated for portable remote terminals.

[0041] However, it may be decided that the geographic location of theremote terminal 102 is not specified at all. In this case, the bursttiming alignment for the extreme cases of the remote terminals, i.e.where the terminal would be nearest to the satellite and where theterminal would be farthest from the satellite, is known or can bedetermined. However, the difference between the two extremes turns outnot to be large. For example, where the remote terminals 102 use analready-synchronized slot and frame counter in a system that includestime-stamps, the remote terminals 102 may generate slot and frame timingthat is delayed from hub timing by D′, where D′ is the hub-to-remoteterminal path delay. The remote terminals 102 may each transmit a linkconnect burst just 2*D milliseconds before its “local” frame start,where D is the average hop delay between the remote terminal and thehub, and is a common parameter to all remote terminals in the network.If all of the remote terminals 102 were to transmit the first linkconnect burst in that manner, the link connect bursts would be receivedat the hub 101 during some period of time that is spread around theframe start. This spread may typically be expected to be not more than,e.g., +/−20 milliseconds. Thus, in this example, it is seen that thelink connect bursts would be expected to occur only around frame startand frame end, regardless of the geographic locations of the remoteterminals 102.

[0042] Consider a frame that is, for example, one second long andcontains one hundred slots that are each ten milliseconds long. Only thefirst two slots and the last two slots in this frame would be expectedto contain a link connect burst, and a good practice would be to avoidallocating resources for normal inbound data at these particular slotswithin the frame. Where the initial frequency error is within +/−F, theportion allocated for the link connect burst would have a width W suchthat at least four slots (in this example) would be allocated within aframe of one hundred slots, and a number of frequency channels thatcovers the 2*F frequency uncertainty would also be allocated, for thelink connect bursts. This is typically significantly lower than theminimal allocation that would be needed for link connect bursts in asystem that does not use time stamps at all (where the width W would bethe entire width of the frame), which demonstrates the benefits of thetime-stamps. This is merely a minimum bound, in this example, for thecapacity that needs to be allocated for link connect bursttransmissions. The system designer may increase this allocation for linkconnect bursts as desired by letting the remote terminals randomly pickfurther delay perturbations from the nominal timing of 2*D millisecondsbefore local frame start at the remote terminal.

[0043] Some remote terminals 102 may be able to minimize the initial+/−F frequency inaccuracy dramatically, thus reducing the minimal spacethat should be kept for first link connect traffic. Such a technique maybe particularly effective, for example, in remote terminals 102 equippedwith phase locked-loop (PLL) synthesizers for synthesizing a frequencyupon which the transmit frequency is based.

[0044] Referring to FIG. 5, an illustrative functional block diagram ofa remote terminal 102 is shown. The remote terminal may include some orall of the elements shown in FIG. 5, including a reference frequencygenerator 501 such as a 10 MHz or other frequency clock, a counter 502,a processor 505 such as a CPU, a demodulator 507, amodulator/synthesizer 503, a block up converter 504, a low-noise-block(LNB) transmitter 508, a tuner 509, a counter 510, and/or varioustransmit logic 511. The remote terminal 102 may communicate with the hub102 via the satellite 103 using an antenna 506 such as a satellite dish.In the illustrative embodiment of FIG. 5, a remote terminal 102generates a frequency F₁ (e.g., in the L-Band range) using a PLLsynthesizer (e.g., synthesizer 503) in an indoor unit (IDU) portion ofthe remote terminal 102. The PLL synthesizer 503 is locked on a localfrequency reference clock 501 F_(ref) (e.g., source 501 where F_(ref)may be 10 MHz or some other frequency). The up-converter 504, which maybe located near the antenna (e.g., 505) in an outdoor unit (ODU) portionof the remote terminal 102, converts frequency F₁ to a transmitfrequency F₂ that is much higher (e.g., C-Band or Ku Band) and that istransmitted over the antenna 505 as the carrier frequency. Theup-converter 504 may use a local oscillator with a constant frequency ofF_(1o). F_(1o) may be locked on the same frequency F_(ref) as the localreference clock. The IDU may transmit F₁ to the ODU by, for example, acoaxial cable, and may multiplex the F_(ref) local reference clock onthat same cable.

[0045] The final transmit frequency F₂ is related to the IDU transmitfrequency F₁, and to the local oscillator frequency F_(1o), by theformula: F₂=F₁+F_(1o). Thus, the final transmission frequency F₂ dependsupon a single local reference frequency source. To transmit at somedesired F₂ frequency, the IDU programs the IDU synthesizer 503 to causeF₁ to be equal F₂−F_(1o). If the frequency F_(ref) of the localreference clock was totally accurate, then F₂ would be produced withouterror. But in reality, the local reference clock frequency F_(ref) isinaccurate. In general, F_(ref)=10*(1+E) MHz (for example, where theideal F_(ref) is 10 MHz), where E is a small signed fractionrepresenting error. Because the transmission chain is entirely locked onthe local reference frequency F_(ref), the final transmit frequency willactually be F₂*(1+E) instead of F₂, so the error will be F₂*E. Onetypical reason for the significant inaccuracy of the reference source inthe IDU is that VSATS are often built to be reasonably priced andtherefore use less expensive electronics than, for example, a hub.Another reason is that certain environmental factors such as temperaturecan affect the remote terminal local reference frequency.

[0046] To correct for errors in the transmit frequency, a “feature rate”of the broadcast transmission emitted from the hub 101 and received bythe remote terminals 102 may be used as a globally-accurate clockreference. The feature rate of the transmission may be the data rate(e.g., bit rate), the baud rate, or the rate of any other feature of thetransmission such as the rate at which MPEG start codes are periodicallytransmitted. Assume, for example, that the data from the hub 101 has adata bit rate of R bits per second. This data rate is typically veryaccurate because of the high quality of equipment used in the hub.Following FIG. 5, a certain predetermined number of pulses (e.g., tenmillion pulses of the local reference clock are counted by the counter502 in order to determine a known period of time, such as a one secondwindow or a window of some other length, related to the local referenceclock. A reconstructed clock (such as a data-rate clock), which is basedon a feature rate of the incoming transmission, of R pulses per secondmay be generated by the remote terminal's demodulator 507. During thisone-second window, the data-rate clock pulses are counted by counter510, measuring the number of clock pulses N of the data clock in thistime window. The data-rate clock pulses may be read by CPU 505. If thefrequency of the local reference clock is inaccurate by a factor of(1+E), then the one-second window is also inaccurate by a factor of1/(1+E), and the number of pulses of the data-rate clock that will becounted will be N=R(1/(1+E)). Since R is known and N is measured, E canbe calculated by the CPU 505. Once the fractional error E is calculated,then the CPU 505 may calculate the correct compensated frequency F₁ thatwould produce a desired F₂. The CPU 505 may make a calculation to offsetthe original F₁ by F₂*E, in order to transmit at a compensated frequencyof F₁−E*F₂, instead of the original erroneous F₁.

[0047] Alternatively, the counted window (e.g., a one-second window) maybe determined by counting a predetermined number of pulses of thedata-rate clock. For example, if it is known that the hub 101 transmitsat a bit rate of R bits per second, then the reconstructed data-rateclock would be R pulses per second, and the remote terminal may count Rpulses of the data-rate clock to determine a very accurate period of onesecond.

[0048] Thus, now that there is an accurate way to measure a certainperiod of time, the local reference frequency F_(ref) may be measured bycounting the number of pulses of F_(ref) over the accurately measuredperiod of time (e.g., one second). By counting the local referencefrequency over the accurately measured period of time, the error E ofthe reference clock F_(ref) may be determined by, e.g., the CPU 505. Saythat the number of pulses of F_(ref) actually counted over the measuredperiod is N pulses. Then, the error E would be E=(N−M)/M, where M is theexpected number of pulses of F_(ref) if F_(ref) were perfect. Once thefractional error E is calculated by the CPU 505, then the correctcompensated F₁ may be calculated by the CPU 505 that would produce adesired F₂. A calculation may be made to offset the original F₁ by F₂*E,i.e. to transmit at a compensated frequency of F₁−E*F₂, instead of theoriginal erroneous F₁.

[0049] Note that the VSAT may count other periods of time less than ormore than one second, such as a half second or two seconds. Moreover,the usage of the reconstructed data clock is merely illustrative. Othertypes of reconstructed clocks may be used, such as a byte clock, MPEGSync, etc.

[0050] Thus, by measuring the ratio of two clocks (the local referenceclock and the reconstructed clock), and by measuring one clock againstthe other, the local reference frequency error may be determined withrespect to a reconstructed clock. In doing so, the synthesized frequencyerror may be compensated for such that the local frequency clock erroris reduced, minimized, or even totally canceled.

[0051] There are many alternative ways to measure the frequency ratiobetween the local reference clock and the reconstructed feature clock.For instance, the demodulator 507 may receive the local reference clockF_(ref), or a clock derived from the local reference clock, such as aclock having a frequency that is a multiple of the local referenceclock. The demodulator 507 may also be programmed with a parameterindicating the known nominal feature rate, such as the symbol rate, forexample, of the incoming transmission. The demodulator 507 may have atracking algorithm, and may track the actual instantaneous value of thefeature rate of the incoming transmission, using the rate of the localreference clock (or the clock derived from the local reference clock).As a result, it may be possible to read from the demodulator a measuredvalue of the reconstructed feature rate. Comparing the nominal featurevalue to the reconstructed feature rate read-out of the demodulatorreveals the local reference clock deviation relative to the actualfeature rate of the incoming transmission. Suppose we read from thedemodulator 507 the value of a reconstructed feature rate Ro, where thenominal feature value is R. Then the fractional frequency error of thereference compared to the feature rate is E=1−Ro/R., and the outgoingtransmission can be compensated accordingly.

[0052] While illustrative systems and methods as described hereinembodying various aspects of the present invention are shown by way ofexample, it will be understood, of course, that the invention is notlimited to these embodiments. Modifications may be made by those skilledin the art, particularly in light of the foregoing teachings. Forexample, each of the elements of the aforementioned embodiments may beutilized alone or in combination with elements of the other embodiments.In addition, aspects of the invention have been pointed out in theappended claims, however these claims are illustrative in that theinvention is intended to include the elements and steps described hereinin any combination or sub combination. It will also be appreciated andunderstood that modifications may be made without departing from thetrue spirit and scope of the invention.

1. In a remote terminal of a satellite network, a method comprising astep of adjusting an outgoing transmission carrier frequency based on afeature rate of an incoming transmission.
 2. The method of claim 1,further including steps of: generating a reconstructed clock from theincoming transmission; counting a predetermined number of pulses of thereconstructed clock to determine a period of time; and counting a numberof pulses of a locally-generated reference clock during the same periodof time, wherein the outgoing transmission carrier frequency is afunction of at least the locally-generated reference clock.
 3. Themethod of claim 2, further including a step of comparing the number ofpulses counted of the reference clock to determine an error, wherein thestep of adjusting includes adjusting the outgoing transmission carrierfrequency to compensate for the error.
 4. The method of claim 2, furtherincluding a step of comparing the number of pulses N counted of thereference clock to determine an error E=(N−M)/M, wherein M is anexpected number of pulses, and wherein the step of adjusting includesadjusting the outgoing transmission carrier frequency based on theerror.
 5. The method of claim 1, wherein the step of adjusting includesupconverting the outgoing transmission carrier frequency to a highercarrier frequency.
 6. The method of claim 5, wherein the higher carrierfrequency is in the C-band or the Ku-band.
 7. The method of claim 1,wherein the outgoing transmission carrier frequency is in the L-band. 8.The method of claim 1, further including steps of: generating areconstructed clock from the incoming transmission; counting apredetermined number of pulses of a locally-generated reference clock todetermine a period time; and counting a number of pulses of thereconstructed clock during the same period of time, wherein the outgoingtransmission carrier frequency is a function of at least thelocally-generated reference clock.
 9. The method of claim 8, furtherincluding a step of comparing the number of pulses counted of thereconstructed clock to determine an error, wherein the step of adjustingincludes adjusting the outgoing transmission carrier frequency tocompensate for the error.
 10. The method of claim 8, further including astep of comparing the number of pulses N counted of the reconstructedclock to determine an error E such that N=R*(1/(1+E)), wherein R is anexpected feature rate of the incoming transmission, and wherein the stepof adjusting includes adjusting the outgoing transmission carrierfrequency based on the error.
 11. The method of claim 1, wherein theincoming transmission is from a hub of the satellite network.
 12. Themethod of claim 1, wherein the feature rate is the baud rate of theincoming transmission.
 13. The method of claim 1, wherein the featurerate is the data rate of the incoming transmission.
 14. The method ofclaim 1, wherein the feature rate is a rate of MPEG blocks in theincoming transmission.
 15. The method of claim 1, further includingsteps of: providing a clock based on a locally-generated reference clockto a demodulator; reading the feature rate value from the demodulator;and comparing the feature rate value read from the demodulator to thenominal feature rate value.
 16. In a remote terminal of a satellitenetwork, a method comprising a step of adjusting an outgoingtransmission carrier frequency based on a comparison of an incomingtransmission with a locally-generated reference clock.
 17. The method ofclaim 16, further including a step of generating a reconstructed clockfrom the incoming transmission, wherein the step of adjusting includescomparing the reconstructed clock with the reference clock.
 18. Themethod of claim 16, wherein the outgoing transmission carrier frequencyis in the L-band.
 19. The method of claim 16, wherein the step ofadjusting includes counting a predetermined number of first pulsesassociated with the incoming transmission and counting how many secondpulses of the reference clock occur during a time required to count thefirst pulses.
 20. The method of claim 19, further including determiningan error based upon a comparison of the number of the second pulsescounted with an expected number of the second pulses.
 21. The method ofclaim 19, further including a step of generating a reconstructed clockfrom the incoming transmission, the first pulses being pulses of thereconstructed clock.
 22. The method of claim 16, wherein the step ofadjusting includes counting a predetermined number of first pulsesassociated with the reference clock and counting how many second pulsesassociated with the incoming transmission occur during a time requiredto count the first pulses.
 23. The method of claim 22, further includingdetermining an error based upon a comparison of the number of the secondpulses counted with an expected number of the second pulses.
 24. Themethod of claim 22, further including a step of generating areconstructed clock from the incoming transmission, the second pulsesbeing pulses of the reconstructed clock.
 25. A remote terminal of asatellite network, comprising: a receiver configured to receive anincoming transmission; and a processor coupled to the receiver andconfigured to adjust a carrier frequency of an outgoing transmissionbased on a feature rate of the incoming transmission.
 26. The remoteterminal of claim 25, further including a modulator configured togenerate the outgoing transmission under control from the processor. 27.The remote terminal of claim 26, further including an upconvertercoupled to the modulator and configured to upconvert the outgoingtransmission.
 28. The remote terminal of claim 25, further including alocal reference clock generator coupled to the processor and configuredto generate a reference clock, wherein the processor is furtherconfigured to generate a reconstructed clock from the incomingtransmission, count a predetermined number of pulses of thereconstructed clock to determine a period time, and count a number ofpulses of the reference clock during the same period of time.
 29. Theremote terminal of claim 28, further including a modulator coupled tothe processor and configured to generate the outgoing transmission,wherein the processor is further configured to determine an error basedon the number of pulses counted of the reconstructed clock, and tocontrol the modulator to adjust the carrier frequency of the outgoingtransmission to compensate for the error.
 30. The method of claim 28,further including a modulator coupled to the processor and configured togenerate the outgoing transmission, wherein the processor is furtherconfigured to determine an error E based on the number of pulses Ncounted of the reconstructed clock such that N=R*(1/(1+E)), wherein R isan expected feature rate of the incoming transmission, and to controlthe modulator to adjust the outgoing transmission carrier frequencybased on the error.
 31. The remote terminal of claim 25, furtherincluding a reference clock generator coupled to the processor andconfigured to generate a reference clock, wherein the processor isfurther configured to generate a reconstructed clock from the incomingtransmission, count a predetermined number of pulses of the referenceclock to determine a period time, and count a number of pulses of thereconstructed clock during the same period of time.
 32. The method ofclaim 31, further including a modulator coupled to the processor andconfigured to generate the outgoing transmission, wherein the processoris further configured to determine an error based on the number ofpulses counted of the reconstructed clock, and to control the modulatorto adjust the outgoing transmission carrier frequency to compensate forthe error.
 33. The method of claim 31, further including a modulatorcoupled to the processor and configured to generate the outgoingtransmission, wherein the processor is further configured to determinean error E based on the number of pulses N counted of the reconstructedclock such that N=R*(1/(1+E)), wherein R is an expected feature rate ofthe incoming transmission, and to control the modulator to adjust theoutgoing transmission carrier frequency based on the error.
 34. A remoteterminal of a satellite network, comprising: a receiver configured toreceive an incoming transmission; a reference clock generator configuredto generate a reference clock; and a processor coupled to the receiverand the reference clock generator and configured to adjust an outgoingtransmission carrier frequency based on a comparison of an incomingtransmission with the reference clock.
 35. The remote terminal of claim34, wherein the processor is further configured to count a predeterminednumber of first pulses associated with the reference clock and count howmany second pulses associated with the incoming transmission occurduring a time required to count the first pulses.
 36. The method ofclaim 35, wherein the processor is further configured to determine anerror based upon a comparison of a number of second pulses counted withan expected number of the second pulses.
 37. The remote terminal ofclaim 34, wherein the processor is further configured to count apredetermined number of first pulses associated with the incomingtransmission and count how many second pulses associated with thereference clock occur during a time required to count the first pulses.38. The method of claim 37, wherein the processor is further configuredto determine an error based upon a comparison of the number of thesecond pulses counted with an expected number of the second pulses. 39.A method in a remote terminal, comprising steps of: transmitting a firstburst; receiving a transmission including correction data responsive tothe first burst; and transmitting a second burst corrected in at leastone of frequency or time in accordance with the correction data, whereinthe remote terminal does not receive time stamp transmissions.
 40. Themethod of claim 39, further including a step of reconstructing slot andframe rates from a feature rate of the transmission.
 41. The method ofclaim 39, wherein the correction data includes at least one of an amountof frequency adjustment or an amount of time adjustment.
 42. The methodof claim 39, wherein the remote terminal is part of a satellite network,the step of transmitting the first burst including transmitting thefirst burst within a predetermined window smaller in both time andfrequency than a frame used by the satellite network.
 43. The method ofclaim 39 wherein the remote terminal is unaware of its geographiclocation.
 44. The method of claim 39, wherein the first burst is a linkconnect burst.
 45. A remote terminal, comprising: a transmission portionconfigured to transmit a first burst; and a reception portion configuredto receive a transmission including correction data responsive to thefirst burst, wherein the transmission portion is further configured totransmit a second burst corrected in at least one of frequency or timein accordance with the correction data, wherein the remote terminal doesnot receive time stamp transmissions.
 46. The remote terminal of claim45, further wherein slot and frame rates are reconstructed from afeature rate of the transmission.
 47. The remote terminal of claim 45,wherein the correction data includes at least one of an amount offrequency adjustment or an amount of time adjustment.
 48. The remoteterminal of claim 45, wherein the remote terminal is part of a satellitenetwork, the transmission portion being configured to transmit the firstburst within a predetermined window smaller in frequency than an entirebandwidth used by the satellite network.
 49. The remote terminal ofclaim 45, wherein the first burst is not aligned to slot and frametiming and does not require knowledge of geographic location.
 50. Amethod in a hub, comprising steps of: receiving a first burst from aremote terminal; and transmitting correction data in response to thefirst burst, wherein the hub does not transmit time stamps.
 51. A hubconfigured to perform the method of claim
 50. 52. A hub, comprising atleast one receiver configured to detect an initial burst transmissionwith a frequency error of at least up to half of a frequency channelspacing.
 53. The hub of claim 52, wherein the initial burst transmissionis not aligned with predetermined frequency channels.
 54. The hub ofclaim 52, further including a transmitter configured to transmitcorrection data in response to the initial burst transmission.
 55. Thehub of claim 54, wherein the hub is further configured to transmit timestamps.
 56. The hub of claim 54, wherein the hub is further configurednot to transmit time stamps.
 57. The hub of claim 54, wherein thecorrection data includes at least one of an amount of frequencyadjustment or an amount of time adjustment.
 58. The hub of claim 54,wherein the hub is part of a satellite network, the initial bursttransmission being transmitted within a predetermined window smaller inboth time and frequency than a frame used by the satellite network. 59.A method in a remote terminal, comprising: a transmission portionconfigured to transmit a first burst; and a reception portion configuredto receive a transmission including correction data responsive to thefirst burst, wherein the transmission portion is further configured totransmit a second burst corrected in at least one of frequency or timein accordance with the correction data, and wherein the remote terminalreceives time stamp transmissions, and slot and frame rates arereconstructed from a feature rate of the transmission.
 60. The remoteterminal of claim 59, wherein the remote terminal is part of a satellitenetwork, wherein the first burst is not aligned with a slot, and isaligned with frame timing with at least some timing error due to adifference of geographic delays within the satellite network, andwherein the remote terminal does not receive external information abouta geographic location of the remote terminal.